Field of the Invention
The present invention concerns a method for operating a magnetic resonance apparatus for recording (acquiring) magnetic resonance data of a patient, wherein a measurement procedure is used in which a number of magnetic resonance sequences, which may be identical, are executed sequentially. The invention also concerns a magnetic resonance apparatus and a non-transitory, computer-readable storage medium encoded with programming instructions for implementing such a method.
Description of the Prior Art
Magnetic resonance apparatuses are widely known and used. In medical imaging, nuclear spins of a patient are aligned in a basic magnetic field and excited by radio-frequency pulses, so that their decay signal can be measured. Gradients are superimposed on the basic magnetic field in order to assign spatial information to the measured magnetic resonance signals.
With the radio-frequency pulses, energy is imparted into the patient, and some of this radio-frequency energy is absorbed and can lead to heating of tissue. Thus safety monitoring in magnetic resonance scanners is common, and energy monitoring represents an important part of this safety monitoring. Such energy monitoring is designed to ensure that tissue is not heated to an unacceptable extent in the magnetic resonance scanner. Therefore, a check is made within the magnetic resonance apparatus, for example by a suitable control device, as to whether, within the framework of a measurement process, at least one predetermined threshold value for the energy imparted into the patient, specified in Joule/Kilogram for example, is being exceeded. This energy monitoring is undertaken by predicting the amount of energy for the intended measurement, for example as part of the preparation of the magnetic resonance sequence, as well as by monitoring the absorbed energy during the measurement process (online monitoring).
Exceeding the threshold value in the preparation of the sequence or during the execution of the measurement process leads to the magnetic resonance measurement being aborted. In such cases, legally-defined threshold values, for example 14,400 Joule/Kilogram can be used as the threshold, but often further manufacturer-specific threshold values, below the legal threshold value, are monitored, which are intended to serve as a warning. If the threshold value serving as the warning is exceeded, a pop-up and/or another message can appear at the operating interface of the magnetic resonance device, which indicates to the operator that the threshold value is about to be exceeded. If this occurs in the sequence preparation, the sequence can still be started by confirming the measurement process, but exceeding the threshold during the measurement process can lead to the measurement process being aborted. A threshold value serving as a warning can be, for example, 6,000 Joule/Kilogram.
The aforementioned threshold values are not reached by many measurement processes that are usually undertaken, even if the processes include multiple sequential executions of magnetic resonance sequences, but problems occur when longer measurement processes are specified in which multiple magnetic resonance sequences are employed sequentially in order to record the magnetic resonance data. An example of this type of process is the type of measurement processes used to monitor a minimally-invasive intervention with the magnetic resonance apparatus, in which, with a specific magnetic resonance sequence or a specific series of magnetic resonance sequences, an imaging area, for example a specific slice, is recorded and displayed continuously for the generation of monitoring images. Examples for such minimally-invasive interventions that are possible under magnetic resonance guidance are biopsies, the positioning of catheters, and the like. By contrast, in diagnostic measurement processes for imaging in order to monitor an intervention at the patient, a continuous updating of the magnetic resonance data is needed. Such continuously performed measurement processes are frequently also interactive, meaning that it is possible to change recording (data acquisition) parameters during the measurement process, for example to adapt a slice to be recorded, or the like.
In order to simplify the workflow for such measurement processes, typically the number of individual measurements to be carried out with magnetic resonance sequences is set to a maximum, the measurement process is started and it is aborted when imaging monitoring is no longer needed. This produces very long “virtual” measurement times, but such measurement that typically are not fully utilized. Frequently an “unlimited measurement time” option is also offered at an operator interface of the magnetic resonance device in order to initiate a continuous measurement process of unknown length.
In this type of operation of many devices for energy monitoring of magnetic resonance apparatuses, it is now established that a limit value for the energy imparted into the patient could be exceeded, so that starting a measurement process is avoided. It is precisely when a lower threshold value serves as a warning that a warning occurs for a majority of these interactive measurement protocols, which makes an acceptable workflow for this application at the magnetic resonance apparatus difficult to impossible.
This is because in the known implementations, the maximum measurements that is able to be used must be reduced manually, often by trial and error, so that the threshold value for the energy input is no longer exceeded. This is a major drawback since the energy input depends on the respective settings of the recording parameters (flip angle, saturation setting, sequence timing etc.). This leads to a complex, manual optimization of a measurement time, which in most cases is just not needed. If generic, maximum allowed values are used as the starting point, these must be valid for all possible measurement protocols (measurement processes) which leads to significant, unnecessary restrictions for individual measurement processes.
This problem arises especially markedly during the described adaptation of recording parameters during the measurement process. Many recording parameters, for example the selection of a slice, have an effect on the actual energy input into the patient, so that a change of a recording parameter related to the slice, for example, can lead to the threshold value being exceeded that is still being determined subsequently for the other given recording parameters, and the measurement then being aborted. This can be problematic, especially when carrying out a minimally-invasive intervention.